Is there any evidence that aphid alarm pheromones work as prey and host finding kairomones for natural enemies?

Abstract

1. The aphid alarm pheromone (E)-β-farnesene (EBF) is often considered to be used by natural enemies as a prey/host finding kairomone. However, studies show opposing results, some appear to confirm an attraction of aphid natural enemies by EBF whereas others do not provide any evidence for the kairomone function of EBF. 2. To clarify if aphid natural enemies are attracted by the amounts of EBF naturally emitted by aphids, the existing literature was reviewed about EBF attractiveness to aphid natural enemies with consideration of the amounts of EBF used in the studies. 3. Thirty-one publications that investigated the ability of EBF, aphid cornicle secretion, and attacked aphids, to attract aphid natural enemies were found. Several studies showed an attraction by EBF, but these used much higher amounts of EBF than usually emitted by aphids during a predator attack. Studies investigating EBF amounts similar to what is emitted by aphids are rare and failed to show attraction. Only two studies document an attraction of natural enemies by attacked aphids. 4. As EBF is emitted in very low amounts, not very stable, and only present after an attack, we suggest that aphid-derived EBF is not a suitable kairomone for most natural enemy species, especially when they are able to use alternative cues. As EBF, amongst other volatiles, is also emitted by herbivore-induced plants, we propose that natural enemies might use plant-derived EBF as a synomone to identify aphid-infested plants via an altered plant volatile bouquet.